feat: case and if for branching in rule synthesis.

[winston-h-zhang]

This PR:

• Implements case and if for branching in rule synthesis.

[porcuquine]

Consider something like this, which is easier for me to follow:

Suggested change

	(defun synthesize-if-segment (if-segment curr-rhs end-handle)
	(destructuring-bind (head if-form &rest branches)
	if-segment
	(assert (eql head 'if))
	(assert (= (length branches) 2)) ; Must only have then and else branches.
	(loop with first = t
	for branch-segments in branches
	for test-segment = (if first `(when ,if-form) `(when (not ,if-form)))
	for curr-rhs-tail = (append curr-rhs (list test-segment))
	when first do (setq first nil)
	append (synthesize-segments branch-segments (copy-list curr-rhs-tail) end-handle)
	into output-rules
	finally (return output-rules))))
	(defun synthesize-if-segment (if-segment curr-rhs end-handle)
	(assert (eql (car if-segment) 'if))
	(destructuring-bind (condition if-branch else-branch)
	(cdr if-segment)
	(flet ((aux (branch condition-form)
	(synthesize-segments branch `(,@curr-rhs ,condition-form)) end-handle))
	(nconc (aux if-branch `(when ,condition))
	(aux else-branch `(when (not ,condition)))))))

[porcuquine]

Did you know you could write the following (for example)?

when (typep kind '(member :rule-binding :restriction))

[porcuquine]

This looks great, but please strongly consider using my suggested simplification of synthesize-if-segment.